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Carburization of niobium- and tantalum-base alloys
JOURNAL OF THE LESS-COMMON METALS
CARBURIZATION
OF NIOBIUM-
I39
AND TANTALUM-BASE
ALLOYS*
H. E. MCCOY Metals and Ceramics Division. (Received
Oak Ridge National Laboratory. Oak Ridge, Temz. (U.S.A.)
July z5th, 1966)
SUMMARY A technique
has been developed
for carburizing
niobium-
and tantalum-base
alloys by exposure to a gaseous benzene-hydrogen mixture. The quantity and distribution of carbon absorbed by the specimen can be controlled by varying the composition of the gas mixture and the temperature of the part to be carburized. Several specimens of Nb-r”/oZr and T-III alloys were carburized and tested in creep rupture to demonstrate
the strengthening
effect of the carbon.
INTRODUCTION The strengthening effects of carbon in refractory alloys is well establishedi. However, the problems associated with the formability and weldability of high-carbon materials in strength additions
are also well-known2. can be brought
Hence, even though we know that basic improvements
about by high-carbon
in order to minimize
contents,
the ill effects of carbon.
we must make only small
The present work shows that
there is another alternative-that of adding the carbon after fabrication. The finished part is made of a basic alloy that contains a strong carbide former such as zirconium. Carbon is added by heating the fabricated part in a hydrogen-hydrocarbon mixture. By varying the temperature and composition of the mixture, it is possible to add varying quantities of carbon and also to distribute it to the desired depth in the material. This technique of carburizing is described and the results of several creeprupture tests are reported temperatures.
to illustrate
the strengthening
effect of carbon
at elevated
EXPERIMENTAL METHODS Test materials The materials used in this study were commercial heats of Nb-1% Zr and T-III (Ta-8% W-2% Hf). The material was obtained in the form of nominal o.obo-in. sheet and the as-received chemical analysis is given in Table I. Both materials contained some residual cold working from the processing. * Research sponsored Carbide Corporation.
by the U.S. Atomic
Energy Commission
J.
Less-Common
under contract
Metals, I2
with the IJnion
(1967)
139-145
140
H. E. MCCOY
Carburization
method
The technique of carburization used in this study is illustrated in Fig. I. The carburizing agent was a hydrogen-benzene mixture which was formed by passing hydrogen over a small vial of solid or liquid benzene. The temperature of the benzene was controlled by an alcohol-dry-ice bath. The mixture of hydrogen and benzene was passed over the heated refractory alloy. In practice, the carburizing furnace would TABLE CHEMICAL
I ANALYSIS
Element
OF TEST
MATERIALS
Content (wt. %)
Zr 0 N H C W Hf
N&I % Zr
T-III
0.93 i 0.0150 0.0055 0.0004 0.0035 n.a. n.a.
n.a. 0.0053 o.oozg 0.0004 0.0070 7.5 I.9
0.05
n.a. = not analyzed. GAS BURN ALUMINA
TEST
Fig. I. Schematic
REACTION
SPECIMEN
OFF CHAMBER
SUS
diagram of apparatus
for carburizing
refractory
metals.
probably take on varied designs to facilitate carburizing a variety of shapes. The exit gas Was then burned. The chamber was evacuated after the desired level of carbon was reached to remove the residual hydrogen from the metal. The temperature of the furnace around the material to be carburized and the concentration of the hydrogenbenzene mixture were parameters that could be varied to control the amount and penetration of the carbon added. The concentration of the benzene was controlled by the temperature of the alcohol-dry-ice bath. Creep-rupture
tests
Creep-rupture tests were run in vacuum at 1200°C. The vacuum was approximately I x 10-7 torr and post-test chemical analyses have shown that the technique used is sufficient to prevent interstitial contamination in excess of IOO p.p.m. during a 1000-h creep test. The details of the testing procedure have been described previously3. J. Less-Common Metals, 12 (1967) 139-145
CARBURIZ.4TION
OF NIOBIUM-
EXPERIMENTAL
RESULTS
AND
TANTALUM-BASE
141
ALLOYS
Carburization Several tabs of Nb-rq/, Zr were carburized carburizing technique. Sections of the specimen
to study the effectiveness of the were analyzed to determine the
carbon content and were examined metallographically to determine the depth of carbon penetration. Figures z, 3, 4 and 5 show micrographs of specimens carburized at 1200°, 1300°, 1400”, and ISOO’C, respectively. Surface layers were formed on the specimens at all temperatures but the depth of carbon penetration (as evidenced by metallographic observation) varied from a few mils at IZOO’C to throughout the specimen at 15oo’C. The carbide film could have been prevented temperature of the benzene to reduce the carburizing potential.
by lowering
the
Fig. 2. Photomicrograph of Nb-I y0 Zr specimen (0.1340 wt. “/o C) exposed to Hz-&H6 (-78°C) for 63.6 h at IZOO’C. Etchant: 250 ml HsO. 60 g NaOH, 20 g tartaric acid, 50 ml lactic acid, 30 ml of 30:/b H$&. (x
(2) Above a certain benzene temperature the amount of carbon absorbed is greater for the higher specimen temperature. This probably indicates that the rate of carbon absorption is controlled by diffusion from the surface into the metal. (3) For equivalent conditions, Nb-1:/b %r absorbs more carbon than T-III. Some gas samples were taken from the test chamber in an effort to determine
H. E. MCCOY
142
Fig. 3. P~otomicrograph of NbI y0 Zr specimen (0.179 wt. “/o C) exposed to Hs-C~HS (-78°C) for 42.3 h at 13oo’C. Etchant: 250 ml HsO, 60 g NaOH, zo g tartaric acid, 50 ml lactic acid, 30 ml of 30% H20z. (x IOO)
Fig. 4. Photomicro~aph of N~-I% Zr specimen (0.231 wt. y0 C) exposed to H2-CaHs (-78°C) for 19.5 h at r400°C. Etchant: 250 ml H20, 60 g NaOH, 20 g tartaric acid, 50 ml lactic acid, 30 ml of 30% HzOz. (x IOO) J. Less-Common
Metals,
IZ (1967) 139-145
Fig. 5. Photomicrograph of a Nb-1% Zr specimen (1.17 wt. % C) exposed to Hz-CaHs (-78°C) for 63.8 h at 1500°C. Etchant: 250 ml HzO, 60 g NaOH, 20 g tartaric acid, 50 ml lactic acid, 30 ml of 30:/o HzOz. (x IOO) 0.10
0.09
0.08
0.07 D
I
5 ‘z 0.06
~Nb-fZr.~200”C
’
P F? u 0.05 i 8 s u 0.04
0.03
ib-iZ; -200
I -160
-120 BENZENE
Fig. 6. Carburization
I -80
I -40
TEMPERATURE
of T-I I I and Nb-I
01
I 0
40
80
(“C)
oh Zr in HZ-CGHB mixture.
H. E. MCCOY
I44
the composition of the effluent gas. With a benzene temperature of o”C, the gas consisted of 11.8% CHJ and 87.9% Hz. This is in fair agreement with the vapor pressure of benzene at this temperatures. Creep-y!hre
tests
The results of creep-rupture tests on carburized Nb-1% Zr specimens are given in Table II. The first group of specimens was tested in the as-carburized condition. Since the carburization was carried out at IZOO”C, the carbon was present primarily as a surface film. Over the range of carbon contents of 0.0035-0.045 wt.%, the rupture life was increased by a factor of 4 and the minimum creep rate was reduced by a factor of IO. The second group of specimens was annealed for I h at 15oo’C after carburizing, so the carbon would have been distributed more uniformly. Over a similar range of carbon concentrations the rupture life was increased by a factor of TABLE
II
RESULTS
OFCREEPTESTSON
Test number
Carbon
Time
content (wt. %)
I%
N~I%
CARBURIZED
Zr*
to indzcated strain (h) 2%
5%
10%
Rupture
Minimum
Elongation
creep rate
(%)
(%lh)
As-carburized 2600 5155 5060
0.0035 0.0050
0.75 1.8
0.022
5103
0.045
4.5 8.0
4.0 5.0 II 42
15 15 31 123
31 32 62 204
96.9 97.5 136.5 361.1
18
60 70 308 1065
107 112
233.4 221.0
0.072
30 170 3oo
430 1410
537.5 1459.8
o.oogo
0.293
0.300 0.152 0.032
87.7 106.8 52.5 48.9
Annealed I h at 1500 OC 2555 5162 5184
0.0035 0.0070 0.027
;z
5256
0.054
30
* Test conditions:
TABLE
5.0
0.054
80.2 86.3
0.0015
32.3 14.5
1204’C and 4000 p.s.i.
III
RESULTS OF CREEP TESTS ON CARBURIZED T-I I I *
Test number
Time to indicated strain /h J
Carbon content (wt. %)
I
1%
Minimum
,
Elonaatzon (%)‘
2%
5%
10%
Rupture
creep rate (%lh)
32.8 80.1 107.5
0.40 0.13 0.065
46.5 52.0 49.4
0.28
47.0
0.07 0.033 0.030
50.3 51.5 35.0
As-carburized 535’ 5350 5349 Annealed
0.0045 0.018
0.7 3.5
1.8 13
9.2 31
0.015
3.0
I9
45
I7 48 68
IO
21
30 49 59
s; 83
I h at 1500°C
5342
0.0030
0.3
5414 5361 5375
0.006 0.014 0.018
1.0 12 IO
* Test conditions:
J. Less-Common
2.0 15 29 33
1204~C and 30,000 p.s.i.
Metals, 12 (1967) 139-145
-
41.4 105.4 71.8 117.4
CARBURIZATION OF NIOBIUM- AND TANTALUM-BASE ALLOYS about 6 and the minimum rupture
ductility
creep rate was decreased
was reduced
I45
by a factor
of 50. However,
the
significantly.
The results of tests on carburized T-III are given in Table III. The carbon content varied only over the range of 0.0030-0.0180 wt.?,,. In the as-carburized condition the rupture life was increased by a factor of 3 and the minimum creep rate was reduced by a factor of 6. ilfter greater
annealing
I h at 15oo’C,
carbon content
had a slightly
influence.
CONCLUSIOiXS This study has shown that carbon can be added to Nb-IO/ Zr and T-III alloys by annealing in a hydrogen-benzene mixture. Various concentrations of carbon can be obtained by varying the temperature of the material and by changing the carburizing potential by adjusting the temperature of the benzene. Creep-rupture specimens of Nb-I?/, Zr and T-III effects of carbon.
were carburized
and tested to demonstrate
the strengthening
ACKXOWLEDGEMENTS The author described
is grateful
to B. MCNABB who conducted
in this paper and to H. R. TINCH who carried
The manuscript
was prepared
by the Metals
the experimental
out the metallographic
and Ceramics
Division
Reports
work work. Office.
Particular recognition is given to J. R. WEIR, W. 0. HARMS, H. INOUYE, and D. A. DOCGLAS, Jr., who reviewed this paper and made several helpful comments during the course of this study.
I 1;. J. DRLGROSSO, C. E. CARLSON AND J. J. KAMINSKY, Development of Cb-Z-C alloys, PIV.-IC$54, September 1965. z li. T. BEGLEY AND A. 1. LEWIS, Influence of carbon additions on the workability and mechanical properties of columbium. In D. L. DOUGLAS AND F. W. I
CARBURIZATION
OF NIOBIUM-
I39
AND TANTALUM-BASE
ALLOYS*
H. E. MCCOY Metals and Ceramics Division. (Received
Oak Ridge National Laboratory. Oak Ridge, Temz. (U.S.A.)
July z5th, 1966)
SUMMARY A technique
has been developed
for carburizing
niobium-
and tantalum-base
alloys by exposure to a gaseous benzene-hydrogen mixture. The quantity and distribution of carbon absorbed by the specimen can be controlled by varying the composition of the gas mixture and the temperature of the part to be carburized. Several specimens of Nb-r”/oZr and T-III alloys were carburized and tested in creep rupture to demonstrate
the strengthening
effect of the carbon.
INTRODUCTION The strengthening effects of carbon in refractory alloys is well establishedi. However, the problems associated with the formability and weldability of high-carbon materials in strength additions
are also well-known2. can be brought
Hence, even though we know that basic improvements
about by high-carbon
in order to minimize
contents,
the ill effects of carbon.
we must make only small
The present work shows that
there is another alternative-that of adding the carbon after fabrication. The finished part is made of a basic alloy that contains a strong carbide former such as zirconium. Carbon is added by heating the fabricated part in a hydrogen-hydrocarbon mixture. By varying the temperature and composition of the mixture, it is possible to add varying quantities of carbon and also to distribute it to the desired depth in the material. This technique of carburizing is described and the results of several creeprupture tests are reported temperatures.
to illustrate
the strengthening
effect of carbon
at elevated
EXPERIMENTAL METHODS Test materials The materials used in this study were commercial heats of Nb-1% Zr and T-III (Ta-8% W-2% Hf). The material was obtained in the form of nominal o.obo-in. sheet and the as-received chemical analysis is given in Table I. Both materials contained some residual cold working from the processing. * Research sponsored Carbide Corporation.
by the U.S. Atomic
Energy Commission
J.
Less-Common
under contract
Metals, I2
with the IJnion
(1967)
139-145
140
H. E. MCCOY
Carburization
method
The technique of carburization used in this study is illustrated in Fig. I. The carburizing agent was a hydrogen-benzene mixture which was formed by passing hydrogen over a small vial of solid or liquid benzene. The temperature of the benzene was controlled by an alcohol-dry-ice bath. The mixture of hydrogen and benzene was passed over the heated refractory alloy. In practice, the carburizing furnace would TABLE CHEMICAL
I ANALYSIS
Element
OF TEST
MATERIALS
Content (wt. %)
Zr 0 N H C W Hf
N&I % Zr
T-III
0.93 i 0.0150 0.0055 0.0004 0.0035 n.a. n.a.
n.a. 0.0053 o.oozg 0.0004 0.0070 7.5 I.9
0.05
n.a. = not analyzed. GAS BURN ALUMINA
TEST
Fig. I. Schematic
REACTION
SPECIMEN
OFF CHAMBER
SUS
diagram of apparatus
for carburizing
refractory
metals.
probably take on varied designs to facilitate carburizing a variety of shapes. The exit gas Was then burned. The chamber was evacuated after the desired level of carbon was reached to remove the residual hydrogen from the metal. The temperature of the furnace around the material to be carburized and the concentration of the hydrogenbenzene mixture were parameters that could be varied to control the amount and penetration of the carbon added. The concentration of the benzene was controlled by the temperature of the alcohol-dry-ice bath. Creep-rupture
tests
Creep-rupture tests were run in vacuum at 1200°C. The vacuum was approximately I x 10-7 torr and post-test chemical analyses have shown that the technique used is sufficient to prevent interstitial contamination in excess of IOO p.p.m. during a 1000-h creep test. The details of the testing procedure have been described previously3. J. Less-Common Metals, 12 (1967) 139-145
CARBURIZ.4TION
OF NIOBIUM-
EXPERIMENTAL
RESULTS
AND
TANTALUM-BASE
141
ALLOYS
Carburization Several tabs of Nb-rq/, Zr were carburized carburizing technique. Sections of the specimen
to study the effectiveness of the were analyzed to determine the
carbon content and were examined metallographically to determine the depth of carbon penetration. Figures z, 3, 4 and 5 show micrographs of specimens carburized at 1200°, 1300°, 1400”, and ISOO’C, respectively. Surface layers were formed on the specimens at all temperatures but the depth of carbon penetration (as evidenced by metallographic observation) varied from a few mils at IZOO’C to throughout the specimen at 15oo’C. The carbide film could have been prevented temperature of the benzene to reduce the carburizing potential.
by lowering
the
Fig. 2. Photomicrograph of Nb-I y0 Zr specimen (0.1340 wt. “/o C) exposed to Hz-&H6 (-78°C) for 63.6 h at IZOO’C. Etchant: 250 ml HsO. 60 g NaOH, 20 g tartaric acid, 50 ml lactic acid, 30 ml of 30:/b H$&. (x
(2) Above a certain benzene temperature the amount of carbon absorbed is greater for the higher specimen temperature. This probably indicates that the rate of carbon absorption is controlled by diffusion from the surface into the metal. (3) For equivalent conditions, Nb-1:/b %r absorbs more carbon than T-III. Some gas samples were taken from the test chamber in an effort to determine
H. E. MCCOY
142
Fig. 3. P~otomicrograph of NbI y0 Zr specimen (0.179 wt. “/o C) exposed to Hs-C~HS (-78°C) for 42.3 h at 13oo’C. Etchant: 250 ml HsO, 60 g NaOH, zo g tartaric acid, 50 ml lactic acid, 30 ml of 30% H20z. (x IOO)
Fig. 4. Photomicro~aph of N~-I% Zr specimen (0.231 wt. y0 C) exposed to H2-CaHs (-78°C) for 19.5 h at r400°C. Etchant: 250 ml H20, 60 g NaOH, 20 g tartaric acid, 50 ml lactic acid, 30 ml of 30% HzOz. (x IOO) J. Less-Common
Metals,
IZ (1967) 139-145
Fig. 5. Photomicrograph of a Nb-1% Zr specimen (1.17 wt. % C) exposed to Hz-CaHs (-78°C) for 63.8 h at 1500°C. Etchant: 250 ml HzO, 60 g NaOH, 20 g tartaric acid, 50 ml lactic acid, 30 ml of 30:/o HzOz. (x IOO) 0.10
0.09
0.08
0.07 D
I
5 ‘z 0.06
~Nb-fZr.~200”C
’
P F? u 0.05 i 8 s u 0.04
0.03
ib-iZ; -200
I -160
-120 BENZENE
Fig. 6. Carburization
I -80
I -40
TEMPERATURE
of T-I I I and Nb-I
01
I 0
40
80
(“C)
oh Zr in HZ-CGHB mixture.
H. E. MCCOY
I44
the composition of the effluent gas. With a benzene temperature of o”C, the gas consisted of 11.8% CHJ and 87.9% Hz. This is in fair agreement with the vapor pressure of benzene at this temperatures. Creep-y!hre
tests
The results of creep-rupture tests on carburized Nb-1% Zr specimens are given in Table II. The first group of specimens was tested in the as-carburized condition. Since the carburization was carried out at IZOO”C, the carbon was present primarily as a surface film. Over the range of carbon contents of 0.0035-0.045 wt.%, the rupture life was increased by a factor of 4 and the minimum creep rate was reduced by a factor of IO. The second group of specimens was annealed for I h at 15oo’C after carburizing, so the carbon would have been distributed more uniformly. Over a similar range of carbon concentrations the rupture life was increased by a factor of TABLE
II
RESULTS
OFCREEPTESTSON
Test number
Carbon
Time
content (wt. %)
I%
N~I%
CARBURIZED
Zr*
to indzcated strain (h) 2%
5%
10%
Rupture
Minimum
Elongation
creep rate
(%)
(%lh)
As-carburized 2600 5155 5060
0.0035 0.0050
0.75 1.8
0.022
5103
0.045
4.5 8.0
4.0 5.0 II 42
15 15 31 123
31 32 62 204
96.9 97.5 136.5 361.1
18
60 70 308 1065
107 112
233.4 221.0
0.072
30 170 3oo
430 1410
537.5 1459.8
o.oogo
0.293
0.300 0.152 0.032
87.7 106.8 52.5 48.9
Annealed I h at 1500 OC 2555 5162 5184
0.0035 0.0070 0.027
;z
5256
0.054
30
* Test conditions:
TABLE
5.0
0.054
80.2 86.3
0.0015
32.3 14.5
1204’C and 4000 p.s.i.
III
RESULTS OF CREEP TESTS ON CARBURIZED T-I I I *
Test number
Time to indicated strain /h J
Carbon content (wt. %)
I
1%
Minimum
,
Elonaatzon (%)‘
2%
5%
10%
Rupture
creep rate (%lh)
32.8 80.1 107.5
0.40 0.13 0.065
46.5 52.0 49.4
0.28
47.0
0.07 0.033 0.030
50.3 51.5 35.0
As-carburized 535’ 5350 5349 Annealed
0.0045 0.018
0.7 3.5
1.8 13
9.2 31
0.015
3.0
I9
45
I7 48 68
IO
21
30 49 59
s; 83
I h at 1500°C
5342
0.0030
0.3
5414 5361 5375
0.006 0.014 0.018
1.0 12 IO
* Test conditions:
J. Less-Common
2.0 15 29 33
1204~C and 30,000 p.s.i.
Metals, 12 (1967) 139-145
-
41.4 105.4 71.8 117.4
CARBURIZATION OF NIOBIUM- AND TANTALUM-BASE ALLOYS about 6 and the minimum rupture
ductility
creep rate was decreased
was reduced
I45
by a factor
of 50. However,
the
significantly.
The results of tests on carburized T-III are given in Table III. The carbon content varied only over the range of 0.0030-0.0180 wt.?,,. In the as-carburized condition the rupture life was increased by a factor of 3 and the minimum creep rate was reduced by a factor of 6. ilfter greater
annealing
I h at 15oo’C,
carbon content
had a slightly
influence.
CONCLUSIOiXS This study has shown that carbon can be added to Nb-IO/ Zr and T-III alloys by annealing in a hydrogen-benzene mixture. Various concentrations of carbon can be obtained by varying the temperature of the material and by changing the carburizing potential by adjusting the temperature of the benzene. Creep-rupture specimens of Nb-I?/, Zr and T-III effects of carbon.
were carburized
and tested to demonstrate
the strengthening
ACKXOWLEDGEMENTS The author described
is grateful
to B. MCNABB who conducted
in this paper and to H. R. TINCH who carried
The manuscript
was prepared
by the Metals
the experimental
out the metallographic
and Ceramics
Division
Reports
work work. Office.
Particular recognition is given to J. R. WEIR, W. 0. HARMS, H. INOUYE, and D. A. DOCGLAS, Jr., who reviewed this paper and made several helpful comments during the course of this study.
I 1;. J. DRLGROSSO, C. E. CARLSON AND J. J. KAMINSKY, Development of Cb-Z-C alloys, PIV.-IC$54, September 1965. z li. T. BEGLEY AND A. 1. LEWIS, Influence of carbon additions on the workability and mechanical properties of columbium. In D. L. DOUGLAS AND F. W. I